1. Field of the Invention
The present invention relates to pyroelectric crystals having high figures of merit (p/K) and which are useful for pyroelectric detection applications, and more particularly to alanine substituted triglycine sulfate crystals doped with phosphorous and/or arsenic.
Definitions
As such terms are used in the present invention such terms have the following meanings:
TGS=Triglycine sulfate PA1 DTGS=Deuterated triglycine sulfate PA1 ATGS=Alanine substituted triglycine sulfate PA1 ATGSP=Alanine substituted triglycine sulfate doped with phosphorous PA1 ATGSAs=Alanine substituted triglycine sulfate doped with arsenic PA1 LTGS=Lithium doped triglycine sulfate PA1 MTGS=Manganese doped triglycine sulfate PA1 TGFB=Triglycine fluoroberyllate PA1 DTGFB=Deuterated triglycine fluoroberyllate PA1 1. instability in the ambient atmospheric conditions, PA1 2. a further lowering of the transition temperature, PA1 3. poor mechanical properties, and PA1 4. an inability to polish very thin samples, of about 10 to 20 .mu.m in thickness. PA1 1. unfavorable changes in the dielectric constant and tangent losses (tan .delta.), PA1 2. non-uniformity in the composition of the grown crystal, PA1 3. poor reproducibility in the chemical composition of the crystal, and PA1 4. no appreciable change in the figure of merit.
"Pyroelectric crystal" is a crystal which, if ferroelectric, when held at a temperature below its Curie temperature will experience a modification or incremental change in the polarization of such crystal when the crystal is subject to an increase or decrease in temperature and the resulting change in the polarization of the crystal is known as the "pyroelectric effect" ("First Book of Pyroelectricity", by S. B. Lang; Gordon & Breach, Science Publisher, London 1974).
"Curie temperature", in .degree.C., means the phase transition temperature in a ferroelectric material.
The "figure of merit" of a pyroelectric crystal is the value (p/K) commonly used in the art to evaluate the usefulness of the pyroelectric effect of pyroelectric crystals in devices employed for pyroelectric applications.
The "figure of Merit" of a pyroelectric crystal equals p/K, as reported in units of C/m.sup.2 .degree.K., where p is the pyroelectric coefficient, K is the dielectric constant, C is the change in coulomb, .degree.K. equals the temperature in degrees Kelvin and m.sup.2 equals the area of the pyroelectric crystal in meters squared.
2. Description of the Prior Art
Various ferroelectric crystals such as single crystals of triglycine sulfate have been developed for use in pyroelectric applications. Relatively large TGS single crystals of very good optical quality can be grown from water solution and have found some use in pyroelectric device applications. However, the utility of these TGS crystals is somewhat limited due to the fact that they have a strong tendency to depolarize with time and/or at elevated temperatures, of about 50.degree. C. or higher, which causes instability and deterioration of their most useful properties. Further, an apparent decrease in pyroelectric coefficient P, an increase in dielectric constant K.sub.22 and loss tangent .alpha. at low electrical frequency below Mega Hertz result in a heavy reduction in the figure of merit therefor for pyroelectric detection applications. The resulting p/K is only about 1.1.times.10.sup.-5 C/m.sup.2 .degree.K. (From hereon in the text, an abbreviated form of the pyroelectric figure of merit will be used, i.e., a p/K of 1.1 will be used to represent a p/K of 1.1.times.10.sup.-5 C/m.sup.2 .degree.K. since all of the reported values are in the range of ".times.10.sup.-5 C/m.sup.2 .degree.K."). The TGS crystals also have a relatively low transition temperature or Curie temperature (Tc) of about 49.degree. C. which also limits the use of these crystals for many pyroelectrical applications because the devices employing such crystals cannot be used above such temperatures since the crystals are not effective, as pyroelectric elements, at temperatures above their Tc.
Attempts to improve the properties of TGS single crystals have been made by doping the TGS crystals with various materials, ie. deuterium substitution (DTGS), alanine substitution (ATGS), lithium substitution (LTGS), manganese substitution (MTGS), and a substitution of the sulfate group with selenate (TGSe) and fluoroberyllate (DTGFB).
Other types of crystals, such as single crystals of a solid solution composition type, have also been prepared in attempts to provide single crystals of improved pyroelectric properties. In the TGS family of solid solution composition type crystals there have also been prepared, for example, TGFB:DTGFB crystals, TGSe:DTGS crystals and TGS:TGSe crystals. Although these other single crystals all provide some improvements in pyroelectric properties, such as the reduction of thermal depoling, and an increase in the Curie temperature (Tc), as compared to those of TGS single crystals, such improvements are only obtained at the expense of other useful properties and lead to the following disadvantages:
The most significant property that is used for the purposes of evaluating the usefulness of a pyroelectric crystal is its figure of merit (p/K). The prior art pyroelectric crystals available to date have, at best, a figure of merit of from about 1.1 for TGS to about 1.7 for DTGFB, at 25.degree. C., which is an optimum operating temperature for pyroelectric devices. This relatively limited range of figure of merit values thus places limitations upon the pyroelectric devices in which they are used.
Attempts have also been made to substitute and/or dope single crystal TGS with various cationic impurities as disclosed in U.S. Pat. No. 3,352,906, but the resulting crystals have the following disadvantages, as potential pyroelectric or ferroelectrical materials:
An object of the present invention is to provide for improved single crystal pyroelectrics having improved properties, and in particular, improved figures of merit.
A further object of the present invention is to provide single crystal pyroelectrics having figures of merit of &gt;1.7, and preferably of .gtoreq.2.0.
A further object of the present invention is to provide improved single crystal pyroelectrics which have improved pyroelectric properties and which are easy and inexpensive to manufacture.
A still further object of the present invention is to provide improved single TGS based crystals which have improved figures of merit as well as higher Curie temperatures and lower dielectric constants (K.sub.22) than single TGS crystals.
A further object of the present invention is to provide new single crystal pyroelectrics having such improved figure of merit values as to allow the new crystals to be used for pyroelectric applications for which pyroelectric single crystal TGS has not heretofore been useful, such as for operating pyroelectric devices employing such crystals above 40.degree. C. and closer to the Tc of the crystals; and for use in certain types of infra red sensors such as vidicon, point detectors and charge coupled devices.
These, and other objects of the present invention, are achieved by doping TGS single crystals with alanine and phosphorous and/or arsenic.